1. Field of the Invention
The present invention relates to a printer for mixing and discharging a quantitative medium and a discharge medium, and more particularly, to a printer capable of forming an image of a high resolution as well as enhancing the productivity.
2. Description of the Prior Art
In recent years, especially in business offices, "desktop publishing", i.e., document creation using a computer is widely spread, and a demand has been increased recently for outputting not only characters and graphics but also a color natural image such as a photograph together with characters and graphics. In order to answer such a demand, it has become necessary to print out a natural image of high quality requiring reproduction of halftones.
Moreover, a so-called on-demand type printer is being rapidly spread. This is a printer, in which, according to a control signal outputted according to a recording signal, an ink droplet is discharged from a nozzle and applied to a medium such as a paper and a film only when necessary during a printing. Such a printer has a possibility to be reduced in size and cost.
For discharging an ink droplet, various methods have been suggested. Among them the most popular method employs a piezoelectric device or a heating device. The former is a method for discharging ink by applying a pressure to the ink by deformation of the piezoelectric device. The latter is a method for discharging ink by pressure of foams generated in the ink heated to evaporate by the heating device.
Also, there have been suggested various methods for approximately reproducing gradation steps with the aforementioned halftones by using the on-demand type printer which discharges the aforementioned ink droplet. As a first method, the voltage level or pulse width of the voltage pulse to be applied to the piezoelectric device or the heating device is changed so as to control the size of the droplet to be discharged and to change the diameter of a printed dot.
However, this method has a problem that if the voltage level or pulse width to be applied to the piezoelectric device or the heating device is decreased too much, ink discharging is disabled. Consequently, the minimum droplet diameter has a limitation, decreasing the number of gradation steps which can be expressed and disabling expression of a low concentration. That is, this method is insufficient for printing out a natural image.
A second method does not change a dot diameter but employs a pixel composed of a matrix of, for example, 4.times.4 dots. Gradation expression is realized on this matrix base by using an image processing method such as a so-called dither method and error diffusion method.
This second method also enables to express 17 gradation steps when a single pixel is composed of a 4.times.4 matrix. However, this method also has a problem. For example, if a printing is carried out with the same dot density as in the first method, the resolution is decreased to 1/4 of the first method, and only a rough image can be obtained. That is, this method is also insufficient for printing out a natural image.
In order to eliminate these problems, the inventors of the present invention have suggested a printer disclosed in Japanese Patent Laid-Open Hei 5-201024 and Japanese Patent Laid-Open Hei 7-195682 in which ink is mixed with diluent, i.e., a transparent solvent, at a predetermined mixture ratio immediately before discharging and the diluted ink is discharged from a nozzle onto a recording material. Hereinafter, the term "carrier jet method" will be used to denote a printing method in which ink which is a quantitative medium is mixed with diluent which is a discharge medium and the discharge medium is discharged for carrying out recording. It should be noted that there is not problem in the aforementioned printer if the diluent is assumed to be a quantitative medium and the ink is assumed as a discharge medium.
In such a carrier jet type printer, the quantity of the quantitative medium which is either ink or diluent is changed so as to change the mixture ratio of ink and diluent for controlling the concentration of a liquid mixture droplet discharged, enabling to modify the concentration of a printed dot. That is, the printer is capable of printing out a natural image having a plenty of half tones without deteriorating the resolution.
As such a printer of two-liquid mixing type, there can be exemplified a printer of so-called internal mixing type. This printer includes at least a discharge medium pressure chamber into which a discharge medium is introduced; a discharge medium nozzle which communicates with the discharge chamber; a quantitative medium pressure chamber into which a quantitative medium is introduced; and a connection section which connects the quantitative medium pressure chamber with the discharge medium nozzle. In this printer, the quantitative medium in the quantitative medium pressure chamber is introduced into the connection section where the quantitative medium is mixed with the discharge medium in the discharge medium nozzle, i.e., a liquid mixture is obtained from the quantitative medium and the discharge medium in the discharge medium nozzle, and the liquid mixture is discharged from the discharge medium nozzle.
However, in the aforementioned printer of internal mixing type, there is a problem that during a wait period when no mixing is to be carried out between the quantitative medium and the discharge medium, the quantitative medium is readily dispersed into the discharge medium in the discharge medium nozzle. Moreover, there is a problem that during a mixing-discharging operation for mixing the quantitative medium with the discharge medium, an unnecessary portion of discharge medium flows into the connection section or an unnecessary portion of the quantitative medium flows into the discharge medium.
If dispersion occurs between a quantitative medium and a discharge medium, the discharge medium which is, for example, diluent is gradually colored, whereas the quantitative medium which is, for example, ink is diluted. This affects the concentration of a mixture droplet discharged and it becomes difficult to adjust an accurate concentration gradation.
The aforementioned flow-in of an unnecessary portion of the quantitative medium or the discharge medium is caused as follows, assuming that the quantitative medium is ink and the discharge medium is diluent. When a liquid mixture of a very low concentration is successively discharged, a pressure functions so that the diluent gradually intrudes into the connection section into which ink is introduced. On the contrary, when a liquid mixture of a very high concentration is successively discharged, a pressure functions so that the ink gradually intrudes into the discharge medium nozzle. In the former case, a mixture droplet of a low concentration is discharged when a mixture of a high concentration is to be discharged. In the latter case, a mixture droplet of a high concentration is discharged when a mixture of a low concentration is to be discharged. This makes it difficult to obtain an accurate concentration gradation.
To cope with this problem, the conventional printer employs a one-way valve made by electrofoming or the like, at the boundary between the connection section which is supplied with a quantitative medium and the discharge medium nozzle, so as to prevent dispersion of the quantitative medium and the discharge medium during a wait period as well as to prevent flow-in and mixing of unnecessary portions of the discharge medium and the quantitative medium during a mixing-discharging operation.
However, the aforementioned one-way valve cannot completely shut out the quantitative medium and the discharge medium from each other during a wait period or completely prevent an unnecessary flow-in of the quantitative medium and the discharge medium during a mixing-discharging operation. Thus, there is a difficulty to obtain an accurate concentration gradation. Moreover, employment of such a one-way valve increases the production cost, deteriorating the productivity.
Under these circumstances, there has been suggested a so-called printer of external mixing type as follows. This printer includes a quantitative medium pressure chamber into which a quantitative medium is introduced; a discharge medium pressure chamber into which a discharge medium is introduced; a quantitative medium nozzle which communicates with the quantitative medium pressure chamber; and a discharge medium nozzle which communicates with the discharge medium pressure chamber, wherein the quantitative medium nozzle and the discharge medium nozzle have openings adjacent to each other. A quantitative medium comes out of the quantitative nozzle, seeping along the nozzle opening so as to be brought into contact with the discharge medium introduced to a vicinity of the nozzle opening, so that a liquid mixture is obtained before the discharge medium is discharged from the discharge medium nozzle, thus discharging the quantitative medium and the discharge medium as a liquid mixture.
Because the quantitative medium nozzle and the discharge medium nozzle are formed separately from each other, there is no problem of dispersion of the quantitative medium and the discharge medium during a wait period, and the unnecessary flow-in during a mixing-discharging operation can also be prevented.
As has been described above, in a printer in which a quantitative medium which is, for example, ink and a discharge medium which is, for example, diluent are mixed to be discharged, it is necessary to accurately control the mixing ratio of the ink and the diluent in order to accurately express a gradation step according to an image data.
In order to achieve this, it is necessary that the ink is completely separated from the diluent during a wait state when no mixing is to be carried out between the ink and the diluent. If the ink is in contact with the diluent during the wait state, the ink flows into the nozzle of diluent and the diluent flows into the nozzle of the ink. This adversely affects the mixing ratio of the ink and the diluent for the following dot, disabling to accurately express a gradation step, and it is difficult to obtain a recorded image of a high resolution.
It is indispensable to accurately carry out seeping of the quantitative medium from the quantitative medium nozzle, from which the quantitative medium is pushed out, toward the discharge medium nozzle, from which the discharge medium is discharged, as well as the discharging of the discharge medium from the discharge medium nozzle so as to be mixed with the quantitative medium to be discharged together. For this, it is necessary to surely mix a predetermined quantity of the quantitative medium with the discharge medium in the discharge medium nozzle.